a lot of good points made by the OP.  I just want to focus on the exchange decentralization issue.

I think a peer to peer based distributed exchange would solve a lot of the problems we've seen recently with Mt Gox and all the other exchanges.

The main problem I see with this idea is transferring fiat currency to/from the system to cover trades.

This is one of the services that the current exchanges provide at present. Without this you can't have a working exchange.

Given the present banking and payment transfer setup I can't see how a peer to peer exchange system can possibly interface with the banks.

So you'd need to set up some kind of transfer service for BTC/fiat which everyone would use.

This would either have to be run by a commercial provider or operated as a non-profit service funded via the bitcoin system eg from an automatic levy on each transfer.  

The system would also have to interface with the bitcoin peer to peer exchange.

I'd imagine that technically implementing the above would not be too difficult.  

But you need to have a reliable, safe and accountable operation for the bank/BTC transfers. Maybe a commercial service would be the best solution for this? In that case you would have competing providers offering this facility.